Behind casing fluid flow detection in producing wells using gas lift

ABSTRACT

Methods are provided for locating and measuring the linear flow velocity and volume flow rate of undesired water production behind casing in a producing well operating on gas lift while the well remains in a producing configuration. A well logging tool sized and adapted for passing through production tubing is lowered through the tubing string into the producing zone. The tool contains a source of 14 MEV neutrons and two longitudinally spaced gamma ray detectors. The detector may be placed alternately above or below the neutron source on the tool while maintaining the same source to detector spacing. With the well on production under gas lift the earth formations behind the casing are irradiated with 14 MEV neutrons, either continuously or in bursts, to activate elemental oxygen nuclei comprising the molecular structure of the undesired water flow. The decay of unstable isotope nitrogen 16 produced thereby is detected by the detection of its characteristic gamma rays at the dual spaced detectors. These measurements may then be interpreted in terms of the linear flow rate and volume flow rate of the undesired water flow.

BACKGROUND OF THE INVENTION

This invention relates to well logging methods and apparatus and moreparticularly to nuclear well logging techniques to determine thepresence of undesired water flow in cememt voids or channels behindsteel well casing in a cased well borehole.

Undesired fluid communication along the cased in portion of a wellbetween producing zones has long been a problem in the petroleumindustry. The communication of fresh or salt water from a nearby watersand into a petroleum production sand can contaminate the petroleumbeing produced by the well to an extent that production of petroleumfrom the well can become commercially unfeasible due to the "water cut".Similarly, in near surface water wells used for production of freshwater for city or town drinking supplies or the like, the contaminationof the fresh water drinking supply by the migration of salt water fromnearby sands can also contaminate the drinking water supply to theextent where it is unfit for human consumption without elaboratecontaminant removal processing.

In both of these instances, it has been found through experience overthe course of years that the contamination of fresh water drinkingsupplies or producing petroleum sands can occur many times due to theundesired communication of water from nearby sands down the annulusbetween the steel casing used to support the walls of the borehole andthe borehole wall itself. Usually steel casing which is used for thispurpose is cemented in place. If a good primary cement job is obtainedon well completion, there is no problem with fluid communication betweenproducing zones. However, in some areas of the world where very looselyconsolidated, highly permeable sands are typical in production ofpetroleum, the sands may later collapse in the vicinity of the boreholeeven if a good primary cement job is obtained. This can allow themigration of water along the outside of the cement sheath from a nearbywater sand into the producing zone. Also, the problem of undesired fluidcommunication occurs when the primary cement job itself deteriorates dueto the flow of fluids in its vicinity. Similarly, an otherwise goodprimary cement job may contain longitudinal channels or void spacesalong its length which permit undesired fluid communication betweennearby water sands and the producing zone.

Another problem which can lead to undesired fluid communication alongthe borehole between producing oil zones and nearby water sands is thatof the so called "microannulus" between the casing and the cement. Thisphenomenon occurs because when the cement is being forced from thebottom of the casing string up into the annulus between the casing andthe formations, (or through casing perforations), the casing is usuallysubmitted to a high hydrostatic pressure differential in order to forcethe cement into the annulus. The high pressure differential can causecasing expansion. When this pressure is subsequently relieved forproducing from the well, the previously expanded casing may contractaway from the cement sheath formed about it in the annulus between thecasing and the formations. This contraction can leave a void spacebetween the casing and the cement sheath which is sometimes referred toas a microannulus. In some instances, if enough casing expansion hastaken place during the process of primary cementing (such as in a deepwell where a high hydrostatic pressure is required) the casing maycontract away from the cement sheath leaving a microannulus sufficientlywide for fluid to communicate from nearby water sands along themicroannulus into the producing perforations and thereby produce anundesirable water cut.

There may have been many attempts in the prior art to evaluate andlocate the existance of cement channels. There have also been manyattempts in the prior art to locate and confirm the existance of socalled microannulus fluid communication problems. Perhaps primary amongthese attempts in the prior art has been that of the use of the acousticcement bond log. In this type of logging operation, the amplitude ofacoustic wave energy which is propogated along the casing from anacoustic transmitter to one or more acoustic receivers is examined. Inprinciple, if the casing is firmly bonded to the cement and to theformations, the acoustic energy propogated along the casing shouldradiate outwardly from the casing into the cement and surroundingformations, thereby reducing the amplitude of the casing signal.However, if the casing is poorly bonded to the cement or if the cementis poorly bonded to the formations, a void space exists and the acousticenergy should remain in the casing and arrive at the acoustic energyreceivers at a much higher amplitude than if a good cement bond existedbetween the casing, the cement and the formations.

Acoustic cement bond logging, however, cannot always reliably detect theexistance of a microannulus which can in some instances permitundesirable fluid communication between water sands and nearby producingzones. If the microannulus is sufficiently small and fluid filled, theacoustic energy propagated along the casing may be coupled across it.Yet it has been found that even such a small microannulus can permitundesirable fluid communication between producing zones. Similarly, apoor quality cement job may go undetected by the use of the acousticcement bond log if the cement sheath is permeated by a variety ofchannels or void spaces which are located unsymmetrically about itscircumference. Such channels or void spaces can permit undesirable fluidflow while the main body of cement is bonded well to the casing and theformations thus propagating the acoustic energy satisfactorily from thecasing outwardly through the cement and into the formations. Therefore,such means as acoustic cement bond well logging have been proven to benot entirely reliable for the detection of potential undesired fluidcommunication paths in a completed wall.

Another approach to locating void spaces or channels in the cementsheath in the prior art has been to inject radioactive tracer substancessuch as Iodine 131 or the like through producing perforations into theproducing formations and into any void spaces in the annulus surroundingthe well casing. The theory in this type of operation is that if thetracer material can be forced backward along the flow path of theundesired fluid its radioactive properties may then be subsequentlydetected behind the casing by radiation detectors. This type of welllogging operation has usually proven to be unsatisfactory however,particularly in loosely consolidated sand formations which is preciselywhere undesired fluid communication is most typically encountered.

In particularly permeable formations such as loosely consolidated sands,the producing formation itself can absorb most of the radioactive tracermaterial which is forced through the perforations. Very little, if any,of the tracer material can be forced back along the path of undesiredfluid flow, particularly, if this involves forcing the flow of traceragainst either formation fluid pressure or upward against the force ofgravity. Therefore, such tracer logging techniques for detecting cementchannels or voids behind the casing have usually proven ineffective inthe prior art.

BRIEF DESCRIPTION OF THE INVENTION

The prior art attempts may thus be characterized generally as attemptsto evaluate the cement sheath. The present invention relates to methodsand apparatus for detecting the undesired flow of water itself in cementchannels or voids behind the casing in a producing well. The nuclearwell logging techniques used in the present invention involve theactivation by high energy neutrons of elemental oxygen nuclei comprisinga portion of the undesirable water flow itself. A source of high energyneutrons is placed inside the well borehole opposite the area to beinvestigated for cement channeling or undesired fluid communicationalong the sheath. A source of approximately 14 MEV monoenergeticneutrons is used to irradiate the area with such neutrons. An oxygen 16nucleus upon the capture of an approximately 10 MEV neutron istransmuted to radioactive nitrogen 16. The radioactive nitrogen 16decays with a half life of about 7.1 seconds by the emission of a betaparticle and high energy gamma rays having energies of approximately 6MEV or more. With a sufficiently high flux of 10 MEV neutronsirradiating the undesired water flowing in a cement void, ormicroannulus channel, enough radioactive nitrogen 16 is created in theundesired water flow itself to be detectable at a pair of longitudinallyspaced detectors. This measurement can be used directly to indicate thespeed of flow of the water in the cement channels. Moreover, noveltechniques are developed in the invention for determining the volumeflow rate of water in such cement channels, microannulus or void spacesfrom the degradation of the high energy gamma ray spectrum by Comptonscattering of gamma rays produced by the decay of the radioactivenitrogen 16. The approximate distance from a single gamma ray detectorto the mean center of the water flow path may be thus determined. Yetanother feature of the invention is that by the use of a pulsed, ratherthan continuous, neutron source for the measurements described, a moreaccurate flow detection is provided by reducing the background gammaradiation caused by relatively prompt thermal or epithermal neutroninteractions in the vicinity of the borehole.

Moreover, by first placing a longitudinally spaced high energy gamma raydetector pair above and then by placing the detector pair below theneutron source, fluid flow occurring within and without the casing maybe distinguished with the use of only relatively valid assumptions. Inyet another feature of the invention, the detection of undesired fluidflow in a producing zone under producing conditions is enabled by theuse of small diameter water flow detection apparatus sized to passthrough production tubing and utilizing the same flow detectionprinciples previously discussed. Also, a technique is developed in theinvention for distinguishing undersired behind casing fluid flow in thesame direction as desired fluid flow in an adjacent production tubingstring passing through a producing zone which is being investigated forcement channeling in a multiple completion wall. In this instance, theflow of water both inside the production tubing string passing throughthe zone being investigated and the undesired flow of water in cementchannels or voids exterior to the casing can be in the same directionand yet still be distinguishable. In other techiques utilizing the novelconcepts of the invention, operations are provided for distinguishingwater flow within and without the casing on the basis of its directionof flow in the logging operation.

Finally, additional novel techniques are disclosed herein forconstructing a production profile across the perforated casing zone in aproducing formation to indicate from which casing perforations anyundesirable water cut is being produced. In this instance, theinstrumentation and methods of the present invention are used todetermine quantitatively the flow rates of undesired produced waterinside the casing.

The above objects, features and advantages of the present invention arepointed out with particularity in the appended claims. The invention maybest be understood, however, by reference to the following detaileddescription thereof when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the geometry of a single detector waterflow sonde.

FIG. 2 illustrates schematically the geometry of a dual detector waterflow sonde.

FIG. 3 is a graphical representation of the response of a continuousneutron source water flow detection system under flow and no flowconditions.

FIG. 4 is a graphical representation of the response of a pulsed neutronsource water flow detection system under flow and no flow conditions.

FIG. 5 is a graphical representation showing gamma ray spectraldegradation as the source of gamma rays is moved to a different distancefrom a detector.

FIG. 6 is a graphical representation showing the count rate ratio at twospaced detectors as a function of distance.

FIG. 7 is a schematic cross sectional view of a dual concentric cylindergamma ray detector.

FIG. 8 is a graphical representation of the response of the detector ofFIG. 7 as a function of the distance of the detector from a gamma raysource.

FIG. 9A-C is a schematic representation of the downhole portion of amodular water flow detection sonde according to the invention.

FIG. 10 is a schematic representation of a water flow detection systemin a cased well bore according to the invention.

FIG. 11 is a schematic diagram illustrating the timing and datatransmission format of the water flow detection system of the invention.

FIG. 12A-B is a schematic diagram illustrating a technique of water flowdetection in a producing well; and

FIG. 13A-B is a schematic diagram illustrating a water flow detectiontechnique in a multiple completion producing well.

DETAILED DESCRIPTION OF THE INVENTION

Before considering a detailed description of hardware systems employedto measure the flow rate of water behind the casing according to theconcepts of the present invention, it will be helpful to consider thetheoretical basis for the measurement according to the principles of theinvention.

The techniques of the present invention are predicated upon the creationof the unstable radioactive isotope nitrogen 16 in the stream of waterflowing behind the casing which is to be detected. This is accomplishedby bombarding the flowing water with high energy neutrons having anenergy in excess of approximately 10 MEV. This bombardment can cause thecreation by nuclear interaction of the unstable nitrogen isotope 16 fromthe oxygen nuclei comprising the water molecules in the flow stream, thenuclear reaction being O¹⁶ (n,p) N¹⁶.

Referring initially to FIG. 1, consider a hypothetical downhole fluidtight sonde 14 housing a 14 MEV neutron generator 11 and a gamma raydetector 12. The center of the gamma ray detector 12 is spaced S inchesfrom the center of the neutron source 11. Also, consider a channel ofwater 13 which flows parallel to the axis of sonde 14 and whose centeris R inches from the center of the sonde 14 and which is flowing fromthe neutron source 11 toward the detector 12. It may be shown that C,the counting rate resulting from the decay of the induced radioactivenitrogen 16 activity which is recorded by the detector 12 is given byequation 1.

    C = Σ.sub.o φ .sub.n G K(R) V(e.sup.λa/2v -e.sup.-λa/2v) (e.sup.λb/2v -e.sup.-λb/2v)e.sup.-λS/v                   (1)

where

V = the volume flow of water (in³ /sec)

λ = 0.0936 sec⁻¹ = the decay constant of N¹⁶

a = the effective irradiation length of the water stream as it passesthe source (inches)

b = the effective detection length of the water stream as it passes thedetector (inches)

v = the linear velocity of the water stream (in/sec)

φ_(n) = the neutron output of the source (neutrons/cm/sec)

G = A geometric and efficiency factor of the detector

K(R) = a function dependent upon the distance R (inches) from the centerof the sonde to the center of the water flow

S = the source-detector spacing (inches)

Σ_(o) = (a constant) = N_(o) Pσa/Mλb where N_(o) is Avogodro's number, Mis the molecular weight of water, ρ is the density of water and σ is themicroscopic cross section of oxygen for neutron capture.

Equation 1 may be rewritten as follows:

    C/V = fK(R) (e.sup.λa/2v -e.sup.-λa/2v) (e.sup.λb/2v -e.sup.-λb/2v) e.sup.-λS/v                  (2)

where f = Σ_(o) φ_(n) G

The quantities S, a and b are characteristics of the water flow sonde 14and are measurable or calibratable quantities. Σo is characteristic ofthe physical properties of water, the water flow sonde, and the O¹⁶(n,p)N¹⁶ reaction and can also be measured. If the source and detectorgeometries are held fixed and the neutron output is held constant,equation 2 then indicates that for a given value of R, C/V is a functionof v the linear flow velocity of the water is not a function of thegeometry (i.e. the annulus size, cement channel, etc.) of the waterflow.

Referring now to FIG. 2, consider a second dual detector hypotheticallogging sonde 24 which contains a 14 MEV neutron generator 21 and twogamma ray detectors 22 and 25, spaced S1 and S2 inches from the centerof the neutron source 21. Shielding material 26 is placed between thesource and detector here. Referring to equation 2, the ratio of countingrates recorded in detectors 22 and 25 may be expressed as:

    C.sub.1 /C.sub.2 = e.sup.+λ(S.sbsp.2.sup.-S.sbsp.1.sup.)/v (3)

Solving equation 3 for v, the linear flow velocity, it may be seen that:

    v = λ(S.sub.2 -S.sub.1)/1n(C.sub.1 /C.sub.2)        (4)

in equations 3 and 4, λ = 0.0936 SEC⁻¹, S₂ - S₁ is a known physicaldimension of the sonde 24 and C₁ and C₂ are the measured count ratequantities. Equation 4 then states that the linear flow velocity v canbe determined without any knowledge of the flow geometry or the distanceR measured from the center of the sonde to the center of the water flow23.

In measuring the flow of water within or behind casing the volume flowrate V, rather than the linear flow rate v, is the primary quantity ofinterest. If the volume flow rate V may be accurately determined, thedecision on whether to perform a cement squeeze (or improved cementingjob) to prevent fluid communication between fresh water sands andproducing oil formations may be made. It will be apparent to those ofskill in the art that if the cross sectional area F of the flow rate isknown, such as would be the case for flow within a well casing, then thevolume flow rate is simply given by equation 5 as:

    V = v · F                                         (5)

however, for flow in cement channels behind the casing, F is not knownand is virtually impossible to measure. It is therefore necessary torelate v, the quantity which can be determined from equation 4 to V,using some other parameter than F which may be either measured orestimated within an acceptable degree of accuracy. The parameter R, thedistance from the center of the sonde to the center of the flow, can beused for this purpose.

It is assumed that the neutron flux, φ_(n), that irradiates a givenincremental volume of water decreases in intensity as a function of 1/R²as the volume increment is moved a distance R from the source.Similarly, it is assumed that the radiation detected by the detectordecreases as a function of 1/R² as the distance R increases from thedetector.

Making the above two assumptions, then the term K(R) of equation 2 maybe expressed as:

    K(R) = P/R.sup.4                                           (6)

where R is a calibration constant. Equation 6 arrived at in this manneris only an approximate equation based on the above assumptions. However,laboratory experiments have confirmed that to a good approximationEquation 6 is representative of the behavior of the function K(R).

Using equations 6 and 2, we can write the volume flow rate V as:##EQU1## where Q = p·f_(i) and i = 1 or 2 (representative of the dualdetectors). Equation 7 states that if v is obtained from equation 4 andR is known, or can be estimated, then the volume flow V may be obtainedfrom the count rate recorded in either detector 22 or 25 (1 or 2 ofequation 7) by using the corresponding value of S_(i). Two separatetechniques for determining R will be disclosed subsequently herein.

The foregoing discussion has illustrated that by using a well loggingsonde containing a 14 MEV neutron source and two gamma ray detectorsthat the linear flow velocity v can be obtained independent of the flowgeometry and position of the moving water if the water flow is parallelto the axis of the logging sonde. Similarly, the theory has indicatedthat the volume flow V can be obtained if the cross sectional area F ofthe flow is known (such as would be the case of flow within well casing)or if the distance from the center of the sonde to the center of thewater flow can be measured or estimated with acceptable accuracy. In thecase of water flowing in a cement channel or annulus, an estimate of Rthat is within the cement sheath surrounding the casing would bereasonable.

In considering the applications and limits of water flow detectionbehind casing, it is necessary to examine the accuracy to which v can bemeasured. Recalling that equation 4 is used to compute v and thatequation 4 contains C₁ /C₂ which is the ratio of counts recorded in thenear and far detectors of a flow detection system as illustrated in FIG.2, it should be noted that the ratio C₁ /C₂ has associated with it aninherent statistical error since the nuclear decay process of thenitrogen 16 isotope is statistical in nature. This statistical error inC₁ /C₂ is an inverse function of the magnitude of C₁ and C₂. The errorin the ratio C₁ /C₂ is therefore affected by any parameter which affectsthe magnitude of C₁ and C₂. Parameters such as the source to detectorspacings S₁ and S₂, the distance R from the center of the sonde to thecenter of the flow, the cross sectional area F of the flow, theefficiencies of the gamma ray detectors G, the counting time interval T,the neutron flux output φ_(n) and the background gamma ray countsrecorded under no flow conditions all can effect the measurement. Itshould be noted that although most of these parameters do not appeardirectly in equation 4 and therefore do not effect the magnitude of v,they do affect the accuracy and precision to which v can be measured.

PULSED vs CONTINUOUS NEUTRON SOURCE OPERATION

Referring now to FIG. 3, a typical set of gamma ray energy spectrarecorded under water flow and no water flow conditions is illustrated.The intensity of detected gamma rays at a single spaced detector isplotted as a function of energy in FIG. 3. The 7.12 and 6.13 MEV gammaray photopeaks characteristic of N-γ decay and their corresponding pairproduction escape peaks are well defined under flow conditions. Somepeak structure is also visible, it will be noted, under the no flowconditions. This results from the activation of oxygen 16 in theformation and the borehole in the vicinity of the source and is recordedby the detector even at a spacing of 34 inches as used for the data inFIGS. 3 and 4. This background spectrum also contains radiation fromthermal neutron capture gamma rays from the formation, casing, andsonde. It will be seen that this source of background radiation can beeliminated by pulsing the neutron source in the manner to besubsequently described.

Most prompt neutron caused gamma radiation will occur within onemillisecond after the cessation of a pulse of neutrons. If, for example,the neutron source is turned on for 1 millisecond and gamma raydetection is delayed for 3 milliseconds subsequent to the cessation ofthe neutron burst before the detectors are activated, then the promptneutron caused gamma radiation will decay to a negligible level. By thencounting the oxygen activation induced gamma radiation which remains forapproximately 6 milliseconds, the relatively high level backgroundradiation as illustrated in FIG. 3 may be significantly reduced. Thisentire pulse-delay-count cycle is then repeated approximately 100 timesper second. Of course, it may be desired for other reasons to operatethe neutron source in a continuous mode and this is possible asillustrated by FIG. 3, but is subject to higher background countingrate.

Although the duty cycle of the neutron source under pulse mode operationconditions is only 10 percent in this mode of operation, the neutronoutput while the source is on is approximately a factor 10 times greaterthan the continuous neutron output if the source is operatedcontinuously. Thus, the integrated neutron output is approximately thesame in pulsed and continuous modes of operation. Under pulsedconditions the duty cycle of the detectors is approximately 60 percent(i.e. 6 to 10 milliseconds). If the count acceptance energy windowillustrated in FIG. 3 (approximately 4.45 MEV to approximately 7.20 MEV)used for the continuous mode operation were used for the pulsed modeoperation, the net counting rate from the decay of the unstable isotopeN¹⁶ would be reduced to approximately 60 percent of that for thecontinous mode. However, under pulsed conditions, essentially none ofthe prompt neutron gamma radiation is recorded. Since there is no majorcomponent of element activation radiation other than that from theunstable N¹⁶ isotope above 2.0 MEV, it is possible to widen the countacceptance energy window when using the pulsed mode from approximately2.0 to approximately 7.20 MEV. This change of range of the countingenergy window will thus include additional counts from Comptonscattered, energy degraded, 6.13 and 7.12 MEV gamma radiation due to theoxygen activation and will thereby increase the count rate to offset thelosses due to the approximately 60 percent duty cycle of the detectorsin this pulsed mode of operation. FIG. 4 illustrates dramatically thereduced background effect by using the pulsed mode of operation. In theillustration of FIG. 4 the same source detector spacing (34 inches) isutilized as in FIG. 3 and the broadened counting energy window at thedetector as previously mentioned is utilized.

To summarize, by operating the neutron generator in a pulsed mode ofoperation the magnitude of the signal from the oxygen activationreaction remains approximately the same while the background radiationis reduced substantially by eliminating the recording of prompt N-γradiation. This increase in the signal to noise ratio of the desiredcounting signal reduces the statistical error of the quantity C₁ /C₂.

Equation 2 shows that the counting rate at a detector C varies as e⁻s/v. This indicates that in order to maximize counting rate C and thusminimize the statistical error in the measurement of v, that thedistance to the detector S should be as small as possible. However, inconsidering the two detector flow meter sonde of FIG. 2, equation 4indicates that if the distance between the two detectors (S₂ - S₁)becomes too small, than v becomes insensitive to the ratio of countingrates C₁ /C₂. It is thus necessary to strike a practical compromise inthe selection of the source detector spacings S₁ and S₂ in order tominimize the statistical and non-statistical errors in v. Appropriateexperimental techniques have been derived from determining optimumspacings S₁ and S₂. These spacings for typical pulsed neutron sources asused in the system of the present invention are pointed out with furtherparticularity in the subsequent description of the equipment. While thetheory of operation of the instrumentation is still valid at otherspacings, it will be appreciated by those skilled in the art that thespacing distances given in the following descriptions are not obviouswithout an experimental basis.

TECHNIQUES FOR DETERMINING R

Recalling equation 7, it will be observed that it is possible to measurethe volume flow rate of the water behind the casing provided a techniqueof determining R, the radial distance from the center of the detectorsto the center of the water flow, may be determined or estimated. Forreasons which will become apparent in the subsequent descriptions,sometimes it is not possible to estimate R to the accuracy necessary tobe able to predict the volume flow rate V. However, it is possible tomeasure R by certain techniques which will now be described in moredetail.

The first technique for determining R may be considered a gamma rayspectral degradation technique. Referring now to FIG. 5, two gamma rayspectra resulting from the decay of radioactive nitrogen 16 produced byoxygen activation with a water flow meter sonde of the type contemplatedfor use in the present invention is illustrated schematically. Thespectra of FIG. 5 are taken at the same detector in the flow meter sondeand illustrate the counting rate at the detector resulting from a waterflow whose center is R₁ and R₂ inches from the center of the detector.The broken curve in FIG. 5 illustrates a gamma ray spectrum resultingfrom the decay of radioactive nitrogen 16 and a water flow whose centeris at a distance R equal approximately 2.96 inches from the center ofthe water flow sonde detector. In the illustration of FIG. 5, thus R₂ isgreater than R₁. Also illustrated by the double ended arrows in FIG. 5are two energy range counting windows A and B. Window A includes the7.12 and 6.13 MEV photo and escape peaks from the radioactive nitrogen16 which are primary radiation which reach the detector without Comptonscattering collosions primarily. Window B is a radiation energy windowfor detecting primary gamma radiation which has been degraded in energythrough collosions (Compton scattering).

If C_(A) (R) is defined as the count rate recorded in window A forarbitrary R and C_(B) (R) is the count rate recorded in window B forarbitrary R, it can be seen that:

    C.sub.A (R.sub.2)/C.sub.B (R.sub.2)<C.sub.A (R.sub.1)/C.sub.B (R.sub.1) (8)

for R₂ >R₁

The ratio inequalities C_(A) /C_(B) in equation 8 which result in thismanner are due to the fact that a larger fraction of the primary 6.13and 7.12 MEV gamma radiation is degraded by collosions with theintervening material as the distance R between the activated water flowand the detector is increased. Thus by calibrating a system for waterflow detection in terms of the spectral degradation as a function of theradial distance R, a tool is provided for determining the unknown radialdistance R to the center of flow. This distance R may then in turn beused in connection with equation 7 for quantitatively determining tovolume water flow rate.

Referring now to FIG. 6, the results of an experimental calibration ofthe ratio of counting rates C_(A) /C_(B) which were measured in knowntest flow conditions as a function of R are plotted with the standarddeviation error bars associated therewith. Also plotted in FIG. 6 arethe results of a monte carlo computer calculation for a 6.13 MEV pointgamma ray source at various distances R from a gamma ray detector. Themonte carlo calculations are based on probability theory and are used topredict the uncollided or undegraded gamma ray flux as a function of theradial distance of the source to the detector using the known laws ofphysics concerning the Compton scattering phenomena. It will be notedthat there is excellent agreement between the experimental curve and themonte carlo calculations as the data points of FIG. 6 indicate.

In the two detector water flow sonde to be subsequently described inmore detail, the ratio of the counting rates at the two selected energywindows C_(A) and C_(B) from the near detector can be measured. Thedistance R from the center of the water flow to the center of thisdetector may then be determined by comparing the background correctedcount rates at these two energy windows with the relationshipillustrated in FIG. 6 in order to determine R, the distance from thecenter of the detector to the center of the water flow. The countingrate ratio at the near detector is used for this purpose due to the factthat it will have a higher counting rate and will thus give betterstatistical accuracy. It will be appreciated, however, that thisrelationship will also hold true for the A detector and, if desired, theA detector count rate ratio may be used alternatively or supplementaryto the near detector count rate ratio for this purpose. The countingrates at the two different detectors can be used to compute v the linearflow rate, and then by using the relationship of equation 7, the volumeflow rate V may be inferred once R is determined in this manner.

Referring now to FIGS. 7 and 8 an alternate technique for measuring R isillustrated schematically. In FIG. 7, a cross sectional view of a dualcrystal gamma ray detector is illustrated schematically. This detectorcomprises an inner crystal 71 generally cylindrical in shape andcomprising a sodium or cesium iodide activated detector crystal ofradius r₁ and length L_(I) which is positioned in the center of acylindrical shell crystal 72. The detector crystal 72 also comprises asodium or cesium iodide thallium-activated crystal of the type known inthe art for detecting high energy gamma rays and having an inside radiusr₂ and an outside radius r₃. Two separate photomultiplier tubes can beindependently optically coupled to detector crystals 71 and 72 and usedto detect independently scintillations or light flashes resulting fromthe interaction of the high energy gammy rays with the crystallinestructure so that two separate counting rates C_(O) and C_(I) may bedetected from the two cylindrical detectors 72 and 71, respectively.

Considering the activated water flow radioactive nitrogen 16 as a pointgamma ray source located R inches from the center detector 71, it may beshown that the ratio of counts C_(O) recorded in the outer crystal toC_(I), the counts recorded in the inner crystal, is given by therelationship of equation 9. ##EQU2##

In equation 9, K is a constant which includes a shielding effect of theouter crystal on the inner crystal for the gamma ray flux. If equation 9is numerically integrated as a function of R using the dimensions givenon the drawing of FIG. 8, a curve such as the solid curve of FIG. 8 isobtained.

FIG. 8 illustrates a graphical representation of the ratio C_(O) /C_(I)as a function of R, the solid curve using the dimensions shown on thefigure. It can be seen from FIG. 8 that R may be obtained from the ratioC_(O) /C_(I) if this ratio can be measured with sufficient accuracy. Thetwo dotted line curves in FIG. 8 comprise the envelope of ±2 percentaccuracy in determining the ratio C_(O) /C_(I) and illustrate the factthat R may be determined to within 1/2 inch if R is less than or equalto five inches by measuring the ratio C_(O) /C_(I) to the accuracy ±2percent. If it is desired to maintain better than ±1/2 inch accuracy inthe measurement of R, then a longer counting interval is required inorder to obtain the ratio C_(O) /C_(I) to an accuracy of better than 2percent.

Summarizing this technique for measuring volume flow rate V, thedetection of the volume flow rate V may be obtained from therelationship of equation 7 provided that the water flow may be eitherestimated or measured by either of the foregoing described techniques.The linear flow rate v is obtained in the manner previously described.Under some water flow conditions R may be measured with accuracy by oneof the foregoing techniques and then used in order to compute V, thevolume flow rate. In some instances, it may be necessary to estimate R.This may be done by assuming that the water flow is in a channel or voidin the cement annulus surrounding the casing outside the well borehole.In such a case, R could be estimated to be 1/2 to 1 inch greater thanthe known casing O.D. In this case the volume flow of water may then besimilarly obtained from the relationship of equation 7. In eitherinstance, techniques for determining the linear flow rate v and aquantitative measure of the volume flow rate V of the water in a cementchannel or annulus void behind the casing in a well borehole have beendescribed in the foregoing sections. The following sections will deal inmore detail with the water flow detection systems and with operationalmeasurement techniques which may be used under different borehole andproducing conditions for detecting and measuring water flow inside oroutside of casing in a well borehole.

DESCRIPTION OF THE EQUIPMENT

The equipment used to make the water flow measurements previouslydiscussed relies on the activation of the oxygen 16 nuclei by thecapture of neutrons with energy equal to or greater than 10 MEV. Thisnecessitates the use of a neutron generator which can generate asufficient intensity of neutrons having an energy of 10 MEV or greaterto perform the measurement. The most commonly available such neutrongenerators are those relying on the deuterium-tritium reaction togenerate this flux of high energy neutrons at a sufficient intensity ofperform measurements of this type. The deuterium-tritium reactionneutron generators are generally referred to as accelerator type neutronsources.

Accelerator type neutron sources generally comprise an evacuatedenvelope having a target materials at one end thereof which isimpregnated with a high percentage of tritium. This target is kept at ahigh negative potential (approximately 125 KV) with respect to thesource of deuterium nuclei which are to be accelerated onto it. At theopposite end of the evacuated container is an ion source and a source ofdeuterium nuclei usually termed replenisher. In operation, suchaccelerator sources generate a concentration of deuterium ions from theion source which are focused by electrostatic lenses into a beam andaccelerated by the high negative potential onto the target materialwhich is impregnated with the tritium nuclei. Due to the highacceleration voltage, the electrostatic Coulomb repulsion between thedeuterium nuclei and the tritium nuclei is overcome and thethermo-nuclear fusion reaction takes place generating a relatively highintensity of neutrons having an energy of approximately 14 MEV.

In constructing the equipment to perform the water flow measurementspreviously discussed, since it is necessary to use an accelerator typeneutron source, a problem arises in the physical construction of thedownhole portion of the system. This problem is caused by the fact thata high voltage power supply is necessary to generate the approximately125 KV potential required by the neutron source for the acceleration ofthe deuterium ions. Perhaps the most efficient such high voltage powersupply is a multiple stage Cockroft-Walton Voltage Multiplier Circuit. Acircuit arrangement for generating a high voltage such as that requiredby the accelerator tube when placed in a well logging instrumentrequires considerable longitudinal extent in order to stack the voltagemultiplying stages longitudinally along the length of the well logginginstrument while providing sufficient insulation about these voltagemultiplying stages to prevent voltage breakdown of the insulators.

Referring now to FIGS. 9A, 9B, and 9C, the downhole sonde for the waterflow detection measurement is illustrated schematically. The sonde ismade up of several component sections which may be physically rearrangedto perform steps in the detection of the water flow behind the casingaccording to the principles previously discussed. The upper end of thesonde is provided with a head member 91 approximately 10 inches inlongitudinal extent. A control and detector electronics section 92 isattached to the head section and is approximately 75 inches inlongitudinal extent. The detector section 93 houses two gamma raydetectors which may comprise thalium activated sodium iodide crystaldetectors (approximately 2 inch by 4 inch cylinders in appearance) andan iron shielding member which is interposed on the end opposite theneutron generator. Below the detector section in FIG. 9A is the neutrongenerator and power supply section housing the neutron generator 94 andthe 125 KV high voltage power supply 95. The spacings preferred betweenthe neutron source and the detectors in the assembled instrument are,respectively, 23 inches and 42 inches as shown in FIG. 9. The neutronsource and power supply section is approximately 94 inches in length.Finally, at the lower end of the sonde is a protective bull plugassembly 96 which serves to protect the lower extremity of the sondeshould it come into contact with the bottom of the borehole or someobstruction therein.

The problem which arises is due to the longitudinal extent (94 inches)of the high voltage power supply. It will be realized by those skilledin the art that in order to detect water flow in an upward directionthat the flow must first pass the neutron source and then subsequentlypass the detectors in its movement. This implies the configurationillustrated in FIG. 9B where the detector section 93 of the well logginginstrument is placed above the high voltage power supply and neutrongenerator section 94 and 95. However, in order to detect water flow in adownward direction, the configuration illustrated in FIG. 9C is requiredwherein the downward water flow must first pass the neutron source andthen pass the gamma ray detectors in order to make the flow measurementas previously described. In this configuration, the neutron source-powersupply section 94, 95 must be placed above the detector section 93 onthe downhole instrument.

Since the gamma ray detectors must be located within a reasonabledistance of the neutron generator target, the tritium impregnated targetof neutron source 94 must be located as close as possible to the shieldportion of the detector section 93 of the instrument. This requires thedesign of a neutron source 94 power supply 95 section which isreversible (i.e. connectable to operate from either end) when going fromthe configuration shown in FIG. 9B to that shown in FIG. 9C in order todetect water flow in an upward or a downward direction, respectively.Similarly, all of the component portions of the downhole instrument ofFIG. 9 are constructed in a modular fashion. These modules may be joinedby screw type fluid tight assemblies and sealed against the incursion ofborehole fluid by sealing means at each of these junctions.

The downhole sonde illustrated schematically in FIG. 9A, B, and C isalso provided with centralizer members 97 which may comprise cylindricalrubber arms or the like which extend outwardly into touching engagementwith the inside walls of the well casing when the sonde is lowered intothe borehole for measuring purposes. These centralizer arms 97 maintainthe body of the sonde in a central position within the casing in orderto assist in preserving cylindrical symmetry of the measurements. If thesonde were able to lie against one side of the well casing, it couldfail to detect water flow on the opposite side of the casing memberbecause of a lack of sensitivity due to the increase distance from theneutron source and detectors to the flowing water.

The electronics section 92 of the downhole sonde functions, as will bedescribed in more detail subsequently, to control the operation of theneutron source 94 and to furnish high voltage power for the operation ofthe detectors which are contained in the detector section 93 of thesonde. The electronics section 92 also serves to provide synchronization(or sync) pulses at the beginning of each neutron burst. The electronicssection 92 also contains circuit means to transmit electrical pulsesignals from the detectors and sync pulse signals up to the well loggingcable to the surface.

Referring now to FIG. 10, a well logging system in accordance with theconcepts of the present invention is shown in a borehole environmentwith the surface equipment portion thereof and is illustratedschematically. A downhole sonde 104 which in practice is constructed inthe modular manner illustrated with respect to FIGS. 9A, 9B, and 9C, issuspended in a well borehole 100 by an armored well logging cable 111and is centralized by centralizers 105 as previously described withrespect to the interior of the well casing 102. The cased borehole isfilled with a well fluid 101. The downhole sonde of FIG. 10 is providedwith dual gamma ray detectors 124 and 125 which are shown mounted in theconfiguration shown in FIG. 9C for detecting water flow in a downwarddirection behind the casing 102. The downhole sonde is also providedwith a 125 KV power supply and neutron generator 126 of the typepreviously described. The electronics section 127 of the downholeinstrument 104 corresponds to electronics section 92 of FIGS. 9A, 9B,and 9C.

Earth formations 123, 107, 108, and 109 are penetrated by the borehole100. A cement channel 110 on one side of the cement sheath 103 of thecased wellbore is illustrating allowing undesired fluid flow in adownward direction from a water sand 107 which contaminates a producingsand 109 separated from the water sand 107 by a shale layer 108. Withthe well logging instrument 104 placed in the position shown and withthe detector source configuration illustrated in FIG. 10, the instrument104 is capable of detecting undesired water flow from the water sand 107through the cement channel 110 into the producing sand 109. Perforations106 in the casing 102 allow fluid from the producing sand to enter thewell borehole 100 as well as allowing the undesired water flow down thecement channel 110 to enter the borehole 100. In the configuration shownin FIG. 10, high energy neutrons from the neutron source 126 penetratethe steel casing 102 and activate the elemental oxygen in the water flowfrom water sand 107 through cement channel 110. The water flowing inchannel 110 then continues past detectors 124 and 125 sometime later andgamma rays resulting from the decay of the radioactive nitrogen 16 aredetected in the manner previously described by the detectors 124 and125. Electrical pulses whose height is proportional to the energy of theimpending gamma rays detected by the detectors 124 and 125 aretransmitted to the electronic section 127 of the downhole instrument andfrom there coupled to the well logging cable 111 conductors andtransmitted to the surface in a form which will be described in moredetail subsequently.

Referring now to FIG. 11, a timing chart for the instrumentation of FIG.10 is shown together with the pulse wave forms appearing on the loggingcable 111. The electrical pulse signals representative of the energy ofthe gamma rays at the detectors 124 and 125 are illustrated in the topportion of the drawing of FIG. 11 while the lower portion of the drawingof FIG. 11 is a schematic representation of the timing involved in theoperation of the system of FIG. 10. It will be observed as previouslydescribed, that a 1 millisecond duration neutron burst is initiated attime T = 0 and extends through time T = 0 plus 1 millisecond.Simultaneously with the initiation of the neutron burst in the downholeinstrument, a large amplitude negative polarity synchronization (orsync) pulse is generated by the electronic section 127 of the downholeinstrument and coupled to the conductors of well logging cable 111. Theamplitude of the sync pulse is made greater than any possible data pulseamplitude from the detectors. Electrical pulse signals representative ofrandomly occurring gamma rays impinging upon detectors D1 and D2 in thedownhole instrument 104 are coupled continuously to conductors of thewell logging cable 111 for transmittal to the surface by the electronicsection 127 also. The pulses from detector D1 are applied to the cableconductor as negative polarity voltage pulses while pulsesrepresentative of the gamma rays detected at detector D2 are applied tothe cable conductor as positive polarity voltage pulses. At the surfacea pulse separator 115 is used to discriminate the detector D1 pulsesfrom the detector D2 pulses on the basis of their electrical polarity.The negative polarity pulses are supplied as input to a synchronizationpulse detector 118 and as one input to a time gate 116. The positivegoing pulses from detector D2 are supplied as one input to a time gate117.

The synchronization pulse detector 118 detects the large amplitudenegative sync pulses on the basis of amplitude and supplied conditioningpulses to the time gates 116 and 117 beginning at a time 4 millisecondsafter the initiation of the neutron burst. Thus, there is a 3millisecond time interval between the end of the neutron burst and theconditioning of time gates 116 and 117 by the synchronization detectorand timing pulse generator circuit 118.

The output of both detectors D1 and D2 are continuously supplied to thewell logging cable 111 but are thus prevented from reaching subsequentcircuitry by the action of time gates 116 and 117 which allow therandomly occurring data pulses to reach the processing circuitry onlyduring the 5.85 millisecond duration interval beginning at 4milliseconds after T = 0 and extending until 9.85 milliseconds after T =0 as illustrated in the timing chart of FIG. 11.

When time gates 116 and 117 are enabled by the conditioning pulse fromsync pulses detector 118, the data pulses from the downhole detectorpair 124 and 125 are coupled as inputs to pulse height analyzers 119 and120 respectively. These pulse height analyzers perform the spectralenergy separation of gamma rays detected by the downhole instrument 104at each of the detectors 124 and 125 according to the energy windowspreviously described. Thus the spectral degradation technique previouslydescribed may be used to derive the distance R from the center of thedetector to the center of the flowing water in the cement channel 110 bythe method previously described with respect to the calibration chart ofFIG. 6. For this purpose, the energy discriminated pulse heightinformation from pulse height analyzers 119 and 120 is supplied to asmall computer 121 which may comprise a general purpose digital computerof the type PDP-11 which is manufactured by the Digital EquipmentCorporation of Cambridge, Massachusetts. The computer 121 may then, whensupplied with the energy discriminated information, apply the countratio technique described previously with respect to the relationship ofFIG. 6 in order to determine R the distance to the center of the waterflow from either or both of the detectors.

It will be appreciated by those skilled in the art that given thepreviously discussed relationships for determining R that such a generalpurpose digital computer may be programmed, for example in a commonlyused programming compiler language such as FORTRAN or the like, toperform the calculations necessary to derive the water flow velocity vand R. Output signals representative of this desired information areconducted from the computer 121 to a recorder 122. The recorder 122, asindicated by the broken line 113, may be electrically or mechanicallycoupled to a sheave wheel 112 in order to display the quantities ofinterest as a function of the depth of the well logging instrument inthe borehole. Similarly, the count information processed by themulti-channel pulse height analyzer 119 and 120 may be conducted to thedata recorder 122 and plotted or displayed as a function of the boreholedepth of the logging instrument 104.

METHODS OF OPERATION

The foregoing descriptions have concerned the theory and equipment whichmay be utilized to detect undesired water flow in cement channels orvoids behind the casing in a cased wellbore. The remaining sectionsdiscuss the methods of operations in various types of wellboreconditions for applying the methods and apparatus which have beendescribed previously. The first such condition for the operation of aflow detection system utilizing the concepts of the present inventionwill concern the operation of such a system in a wellbore wherein theborehole is cased and is producing fluid under formation pressurethrough perforations directly into the wellbore. This situationcorresponds to the borehole described schematically with respect to FIG.10.

DETECTIONS OF UNDESIRED FLOW IN PERFORATED CASING COMPLETION

Referring to FIG. 10, downhole instrumentation which has been previouslydescribed is shown in a casing perforation completion. Undesired waterflow from a water sand 107 is communicated along a cement channel 110past a shale break 108 and into a producing sand 109 where it is allowedto enter the wellbore 100 through casing perforations 106. Although FIG.10 illustrates the case where the undesired water production evolvesfrom downward flowing water from water sand 107 into the producing sand109, it will be appreciated by those skilled in the art that an equallylikely probability is undesired water communication along a similarcement channel (not shown) from a water sand which lies below thehorizon 109. In practice, it will not usually be the case that thedirection from which the undesired water cut is arriving is known withprecision. In fact, it is the purpose of the instrumentation and methodsof the present invention to enable the detection of such channeling orundesired fluid flow from either direction.

It will be recalled that it is necessary to activate the elementaloxygen nuclei comprising the water flow in order to enable theproduction of the radioactive nitrogen 16 whose radioactive decay isdetected by the longitudinally spaced detectors 124 and 125 in thedownhole instrument 104. Since the direction of fluid flow may not beaccurately anticipated, it is therefore necessary to use modularinstrumentation which has previously been described in detail withrespect to FIGS. 9A, 9B, and 9C which may be assembled to detect waterflowing in an upward direction or water flowing in a downward directionbehind the casing.

It has been found through experimental usage of such instrumentationthat such a tool is highly discriminative in its detection of water flowdirection. In practice it has been found that if the instrument isconnected in the manner to detect water flow in an upward direction,that its response to water flow in a downward direction over an intervalof wellbore being investigated is approximately that illustrated withrespect to FIG. 4 for the "no water flow run" of the instrumentationwhen a pulsed neutron source is used or with respect to the "no flowrun" of FIG. 3 when a continuous neutron source is used. Thus, theinstrument has been found to effectively precisely discriminate thedirection of water flow past the neutron source 126 according to whetherthe longitudinally spaced detectors 124 and 125 are placed above orbelow neutron source 126. In order to detect water flowing upwardly, thedetectors are placed above the neutron source and in order to detectwater flowing downwardly, the detectors are placed below the neutronsource.

Bearing this directional discrimination in mind and referring again tothe cased wellbore completion illustrated in FIG. 10, the followingsequence of operations would be required in order to precisely locatedthe undesired water flow or channeling condition illustrated in FIG. 10.First the instrumentation would be connected with the longitudinallyspaced detectors 124 and 125 located below the neutron source 126 inorder to detect water flowing downwardly as shown in the illustration ofFIG. 10. The instrument would then be lowered to a depth slightly abovethe perforated interval 106 and measurements of the radioactive nitrogen16 decay in the downwardly flowing water in cement channel 110 would bemade over a suitable time interval, for example, approximately 5minutes. While the downhole tool 104 is located slightly above theperforated interval 106, it will remain insensitive to any fluid flowwithin the casing 102 in an upward direction as such flow would pass thedetectors 124 and 125 initially and would not pass the neutron source126 prior to passing the detectors. Thus, only the downward flowingwater in the cement channel 110 would be activated and detected by thedownhole instrumentation in this configuration. The modular downholeinstrument is then removed from the wellbore and the source-detectorconfiguration reversed, placing the detectors above the neutron sourceon the body of the well logging sonde as shown in FIG. 9C. Theinstrument is then lowered to a point slightly below the perforations106 in the wellbore and the oxygen activation measurement cycle isrepeated for a suitable time interval. This enables the detection of anywater flowing upwardly along cement channels adjacent to the casing. Inthis configuration, the downhole instrument remains insensitive to anyproduced fluid within the casing 102 moving in a downward direction pastthe detectors 124 and 125.

In this manner, the response of the detectors to any undesired fluidflow along cement channels or voids can be utilized in the relationshipaccording to equation 4 in order to determine the linear flow velocity vof the undesired water flow in the cement channel. The direction of suchflow would, of course, also be defined in this operation.

In a like manner, the volume rate V of any detected undesired fluidmovement may be obtained by estimating or measuring the distance R tothe center of the flow from the center of the detectors by either of thetechniques described previously. If it is not desired to pursue suchmeasurement techniques the approximate volume flow rate V may beestimated by assuming the distance R to be from 1/2 to 1 inch greaterthan the outside diameter of the casing. Then using the relationshipgiven by equation 7, the volume flow rate V may be quantitativelyderived.

The foregoing techniques have been described in terms of stationarymeasurements. Perhaps this is the most accurate form for performing flowdetection according to the techniques of the present invention. It hasalso been experimentally determined that the water flow logging systemof the present invention may be operated with the well logginginstrument in motion. In this case, if the instrument is moved at a slowrate which is accurately known for example, five feet per minute or thelike, the instrument may, in the case of the example of FIG. 10, befirst placed in the borehole with the detectors located below theneutron source and initially located just above the casing perforationsin the area to be inspected. The instrument is then slowly loweredcontinuously past the casing perforations 106 for a predetermined shortdistance below the perforations. Similarly, the downhole tool may thenby removed from the borehole, the detector source configurationreversed, and the instrument lowered to a predetermined position belowthe perforations 106 and moved at a slow rate in an upward directionpast the casing perforations 106. This motion is continued for apredetermined distance above the perforations. In this type ofoperation, when the detectors 124 and 125 are located below the source,the instrument remains relatively insensitive to its motion in adownward direction. With the detectors located above the source, theinstrument remains relatively insensitive to its motion in an upwarddirection. In this manner, it is possible to detect at leastqualitatively by a continuous logging measurement any undesired fluidcommunication along the cement sheath and to record such as a functionof borehole depth in the manner previously discussed with respect to thedescription of FIG. 10.

If it is desired to move the downhole instrument upwardly with thedetectors 124 and 125 located below the neutron source 126 or if it isdesired to move the instrument downwardly with the detectors 124 and 125located above the neutron source 126, then the movement of the toolmerely adds a constant known linear velocity term to the tool responseto water flow in the direction of tool sensitivity. Since the motionspeed of the tool is known prior, this constant term may be compensatedfor by subtracting it out in determining the linear flow velocity v andthe volume flow rate V in the computer system 121 of FIG. 10. Unless thespeed of undesired water flow then were precisely the same as the rateat which the instrument is being moved through the borehole so that norelative motion would exist, it would still be detectable under theseconditions of motion of the instrument.

DETECTION OF UNDESIRED FLUID FLOW IN CEMENT CHANNELS IN A PRODUCING WELLON GAS LIFT

In some instances it may be desired to try to determine the location andamount of undesired fluid flow along cement channels or voids in aproducing well which is completed and in production on gas liftoperation. Such completion techniques are quite common in somegeographical areas where relatively large amounts of natural gas areavailable to assist production. In these instances it is alwaysdesirable to try to measure the undesired fluid flow under producingconditions. This is due to the fact that if production in the suspectedzone is stopped in order to make the measurement of undesired fluidflow, any pressure differentials which existed during production fromthe producing zone and which contributed to the undesired fluid flowwould be lost if the zone were removed from production.

In gas lift operations, a producing zone is generally produced through arelatively small (3 inch) diameter string of production tubing which ispassed through a packer anchored inside the casing at a distance ofgenerally 50 to 60 feet above the producing perforations. Gas liftvalves are installed in the production tubing string above the packerand above the perforations which function to allow production throughthe tubing string when the fluid level is below the valve. These valvesalso permit the introduction of natural gas under pressure into theannulus between the production tubing and the well casing. This pressureis used to force the well fluid up the production tubing string. The gaswhich is permitted to enter the production tubing forms a bubble typeemulsion with the well fluid being produced from the perforations andlifts this to the surface in the production tubing due to the gaspressure.

In gas lift operation, therefore, it is readily apparent that in orderto maintain producing conditions the production tubing string may not beremoved from the wellbore or the action of the gas lift apparatus willbe curtailed. This, of course, would stop production fluid flow andpossibly alter any undesired fluid flow rendering it difficult if notimpossible to detect.

In order to perform undesired fluid flow detection while maintainingproduction on gas lift operations, it is apparent that a well logginginstrument sized and adapted to be passed through production tubing isrequired. Such an instrument may be built having the same configurationas that previously described with respect to FIGS. 9 and 10 and in theforegoing description. That is to say, a neutron generator tube andscintillation detectors which are appropriately sized are placed in aninstrument housing which has an overall outside diameter not exceeding1-11/16 inches. This instrument is then passed through the productiontubing to the desired interval for performing the undesired flowdetection.

Referring now to FIG. 12, the procedure for performing undesired fluidflow detection under gas lift completion conditions is illustratedschematically. A well casing 201 is cemented in place and a producingzone which is producing through perforations 202 is isolated from theremainder of the wellbore by a packer 203 through which a productiontubing string 204 passes to communicate the produced fluid to thesurface. A gas lift valve 205 is provided for appropriately introducingnatural gas under pressure into the production tubing.

In order to detect undesired fluid flow in a downward direction, athrough tubing sized instrument 206 according to the concepts of thepresent invention and having a source-detector configuration asillustrated in FIG. 12A is passed through the production tubing into theinterval just above the producing perforations 202. With theinstrumentation as shown in FIG. 12A, fluid flow in a downward directionmay be discerned according to the previously described techniques in asimilar manner. Similarly, referring to FIG. 12B, if the downholeinstrument 206 is configured with the detectors located above theneutron source and is lowered through the production tubing string intothe producing zone and lowered below the perforations 101, undesiredfluid flow in an upward direction along the casing may be detected inthe same manner as previously described with respect to the larger sizedinstrumentation.

In conducting these measurements, the downhole instrument 206 may eitherbe positioned in a stationary manner first above and then below theperforations, with the detectors located first below and then above theneutron source in the manner previously described, or the instrument 206may be lowered slowly past the perforations in a moving downwarddirection or pulled slowly upward past the perforations in a movingupward direction as previously described. In either event, the operatingprocedures for determining the location, linear flow velocity and volumeflow rate of undesired fluid production in cement channels or voidsexterior to the casing will remain similar to those described withrespect to the foregoing discussions.

DETECTION OF UNDESIRED FLUID FLOW IN MULTIPLE ZONE COMPLETION WELLSUNDER GAS LIFT OPERATION

In multiple completion wells, two or more producing zones located atdifferent depths which are isolated from each other by packers setinside the casing are produced through multiple tubing strings. In suchan instance, naturally the flow from a deeper producing zone must passthrough the shallower producing zone or zones within its productiontubing string. As it is possible that this production from the lowerproducing zone will contain some amount of water cut, the detection ofundesired fluid flow behind casing in the upper producing zone iscomplicated by this factor. The problem therefore arises of how todiscriminate against the detection of the fluid flow containing water inthe adjacent tubing string passing through a shallower producing zonewhich is isolated by packers straddling the perforations in such ashallower zone.

This situation is illustrated schematically in FIG. 13. In the diagramsof FIGS. 13A and 13B, a shallow producing zone 303 is isolated by casingset packers 304 and 305 from the remainder of the wellbore and a lowerproducing zone 306. The lower producing zone 306 is producing under gaslift operation through a tubing string 307 which passes completelythrough the shallower packer isolated producing zone 303. The upperproducing zone 303 is producing through a set of perforations 308 whilethe lower producing zone 306 is producing through a second set ofperforations 309.

In order to allevaite the complexity of the problem of detectingundesired fluid flow behind the casing 310 in the upper zone, the lowerzone could be merely shut in to production during the measurement.However, if the two producing zones are close enough together and theundesired water cut in the upper zone is coming from a water stringerlying between the two zones, this shut in of the lower zone could affectthe well flow conditions in the upper producing zone and thereby renderthe undesired fluid flow measurement undetectable. However, according tothe techniques of the present invention, undesired fluid in the upperregion which is operating on gas lift may be detected in spite of awater cut fluid component being present in the production tubing string307 passing through this zone. The measurement technique for making thisdetermination will, however, require some further theoreticalexplanation.

Recalling the previously discussed theory of the gamma ray spectraldegradation due to the differing thicknesses of scattering materialbetween the source and the detectors for water flow occurring atdifferent distances from the detectors, the count rate C recorded in anenergy region or window i (i = A,B) of detector j (j = 1,2) aftercorrecting for background, may be written as

    C.sub.i,j = C.sup.T.sub.i,j + C.sup.F.sub.i,j              (10)

where in equation 10, C^(T) _(i),j is the count rate from the waterflowing in the production tubing passing through the upper zone andC^(F) _(i),j is the count rate from the water behind the casing in theupper producing zone of FIG. 13. It may be shown that the ratio of thetwo detector count rates due to the tubing flow in energy window A, isas given by equation 11:

    C.sup.T.sub.A,1 /C.sup.T.sub.A,2 = e.sup.K/v.sbsp.T        (11)

where

K = λΔS

Δs is the detector spacing

v_(T) is the linear velocity of fluid flow in the tubing string (in/sec)and

λ = 0.0936 sec⁻¹

Similarly, the ratio of the count rates at the two detectors due to theflow of water outside the casing in energy window A may be shown to begiven by equation 12:

    C.sup.F.sub.1,A /C.sup.F.sub.2,A = e.sup.K/v.sbsp.F        (12)

where v is the linear flow velocity of the undesired water flow behindthe casing and K is as previously defined.

The count rate C_(A),1 may therefore be written as

    C.sub.A,1 = C.sup.T.sub.A,2 e.sup.K/v.sbsp.T + C.sup.F.sub.A,2 e.sup.K/v.sbsp.F                                          (13)

but the count rate C_(A),2 may also be written as

    C.sub.A,2 = C.sup.T.sub.A,2 + C.sup.F.sub.A,2              (14)

by solving equation 14 C^(T) _(A),2 and substituting this into (13) thecount rate C_(A),1 may be written as

    C.sub.A,1 = C.sub.A,2 e.sup.K/v.sbsp.T - C.sup.F.sub.A,2 e.sup.K/v.sbsp.T + C.sup.F.sub.A,2 E.sup.K/v.sbsp.F                          (15)

similarly, an equation may be developed for the count rate in energywindow C_(B),1 which may be written as

    C.sub.B,1 = C.sub.B,2 e.sup.K/v.sbsp.T - C.sup.F.sub.B,2 e.sup.K/v.sbsp.T + C.sup.F.sub.B,2 e.sup.K/v.sbsp.T                          (16)

but here also the count rate C^(F) _(B),2 may be written as

    C.sup.F.sub.B,2 = C.sup.F.sub.B,1 e.sup.-K/v.sbsp.F        (17)

now, substituting equation 17 into equation 16 yields equation 18 asfollows for the total count rate C_(B),1

    c.sub.b,1 = c.sub.b,2 k/v.sbsp.T - C.sup.F.sub.B,1 (e.sup.K/v.sbsp.F + 1) (18)

but also the count rate C^(F) _(A),2 is given by equation 19

    C.sup.F.sub.A,2 = C.sup.F.sub.B,1 L(R.sub.f) e.sup.-K/v.sbsp.F (19)

wherein in the expression of equation 19, L(R_(f)) is a function ofR_(f) the distance between the center of the sonde and the center offlow behind the casing. It will be recalled that this function isillustrated for a particular experimental geometry by the graphicalrepresentation of FIG. 6 which was previously discussed. An approximateanalytical expression for the function L(R) for a particular sondegeometry may then be developed and is given by the expression ofequation 20.

    L(R) = 6.5 - 0.8 R                                         (20)

substituting equation 19 into equation 15 yields equation 21.

    C.sub.A,1 = C.sub.A,2 e.sup.K/v.sbsp.T - C.sup.F.sub.B,1 L(R.sub.f) (e.sup.K/v.sbsp.t e.sup.-K/v.sbsp.F + 1)                  (21)

substituting equation 18 into equation 21 yields expression of equation22. ##EQU3##

Equation 22 may be solved for the unknown function L(R_(f)) which isseen to be given by equation 22-a. ##EQU4##

Similarly, an expression for v, the linear flow rate of the water behindthe casing may be developed as equation 23. ##EQU5##

Referring to FIG. 13A, the distance R_(T) which is measured from thecenter of the sonde 303 to the center of the production tubing 307 isgenerally known or can be estimated to an acceptable degree of accuracy.Equation 20 can therefore be used to compute the function L(R_(T)) fromR_(T). The remaining terms on the right hand side of the equation 23 areknown (K) or are measured quantities (C_(A),1, C_(A),2, C_(B),1, andC_(B),2). Equation 23 can, therefore, be solved for v_(F), the linearflow rate of the water behind the casing. Equation 15 may be rewrittenas equation 23, then as follows: ##EQU6## The term v_(T), which is thelinear flow velocity within the production tubing 307, can be computedfrom the rate of water produced (which is usually known) and the crosssectional area of the production tubing 307. The remaining terms on theright hand side of equation (24) are either known (K), can be computed(v_(F)), or are measured (C_(A),1 and C_(A),2). Equation 23 can,therefore, be solved for C^(F) _(A),2.

Using v_(T) which can be computed as described above and the measuredquantities C_(A),1, C_(A),2, C_(B),1, and C_(B),2, equation 22-a can besolved for L(R_(F)). This value of L(R_(F)) can then be substituted intoequation (20) to obtain R_(F), the radical distance between the centerof the sonde 303 and the center of the flow behind the casing.

Finally, using v_(F) obtained from equation 23, Rf obtained fromequations (22A) and (20) and C^(F) _(A), 2 obtained from equation (24)equation (7) can be used to compute V_(F) the volume flow rate of thewater behind the casing where

C_(i) = C^(F) _(A),2

r = r_(f)

v = v_(f)

and Q is an empirically determined calibration constant.

The foregoing technique is, of course, equally applicable both above andbelow the perforations in the upper producing zone of the multiple wellcompletion so that fluid flow in both an upward and downward directionon opposite sides of the perforation may be detected in this manner.This technique which has just been described may be thought of asdiscriminating against the detection of the known fluid flow within theproducing zone through the production tubing passing from the lower zoneon the basis of its distance from the detectors being different fromthat of any possible undesired fluid flow exterior to the casing.

Of course, it will be appreciated by those skilled in the art that ifthe two producing zones illustrated in FIG. 13 are in reality severalhundred feet apart such that the undesired fluid flow in one zone couldnot conceivably be affected by shutting in the production in the deeperzone, then the more desirable technique would be to simply shut in theflow of production from the lower zone to eliminate the interference dueto the flow of this fluid through its production tubing string whichpasses through the shallower producing zone. However, if as previouslystated, the two zones are not close enough together so that this shut inprocedure would not be desirable in order to preserve the operatingparameters of each producing zone as near as possible, then the justdescribed technique may be utilized to discriminate the flow of fluidexterior to the casing from that in the production tubing string passingwithin the casing.

It will further be appreciated by those skilled in the art that the sametheory would apply whether there are two or even more producing zonescompleted at lower depths than the production zone being investigated.In this case, the above described procedure and theory could be extendedin a similar manner as described above to individually take into accounteach flow contributions from all such production tubing strings whichpass through the zone being investigated.

In making the measurement then, the procedure would be to lower thesmall diameter (1-11/16 inch) sonde through the producing tubing stringinto the producing zone to be investigated. Stationary oxygen activationcount rate measurements would then be taken in the two energy windows Aand B both above and below the perforations in the producing zone withthe detectors first below and then above the neutron source in the samemanner previously described. The foregoing interpretation of these countrates would then be applied. The volume flow rate V and linear flow ratev of any undesired fluid flow behind the casing would thus bedetectable.

PRODUCTION PROFILING IN CASED WELLBORES

While the foregoing discussions have concerned the detection ofundesired fluid flow behind the casing, it is readily apparent to thoseskilled in the art that the instrumentation involved may be used toperform production profiling from spaced perforations within the casing.In such an instance, the downhole sonde would be set up within theneutron source located below the dual spaced detectors and stationarymeasurements taken at intervals between each set of perforations in aproducing zone which is perforated, for example, over a long interval.As the sonde is moved downwardly past each set of perforations, thelinear flow velocity and volume flow rate of the fluid within the casingmay be determined very accurately since the casing cross-section F isaccurately known in detail. As the oxygen activation technique of themeasurement would not detect moving hydrocarbon as it flows past theinstrument but would detect any undesired water cut entering from aparticular set of perforations, then as the sonde is lowered past a setof perforations which is producing the undesired water cut, the volumeflow rate and linear flow velocity of water entering from a particularset of perforations would be detectable by the foregoing techniques.

In this manner, the detection of undesired water stringers in aproducing formation which is perforated over a long interval ofproduction may be located. The technique for determining the linear flowrate v, and volume flow rate V, of the undesired water cut past thedetectors would be the same as previously described.

It is conceivable that an undesired water cut may be produced whichwould flow downwardly within the casing with equal facility to thatwhich would flow upwardly within the casing. In this case, the logginginstrument may be raised to the surface, the source-detector reversed,and the sequence of stationary measurements at locations between theperforations repeated while moving the sonde downwardly past each set ofperforations. In this manner, undesired water cut which is produced froma given level of perforations may be detected within the casing whetherit flows in a downward direction from the peforations or in an upwarddirection from the perforations within the casing.

The foregoing discriptions may make other alternative embodiments of themethods and apparatus of the present invention apparent to those ofskill in the art. It is therefore the aim of the appended claims tocover all such changes and modifications as fall within the true spiritand scope of the invention.

We claim:
 1. A method for measuring the direction, linear flow rate andvolume flow rate of behind casing water migration in a cased portion ofa producing well operating on gas lift and producing well fluid throughcasing perforations which is transmitted to the earth's surface inrelatively small diameter production tubing, without substantiallyinterrupting the production of the well, comprising the steps of:a.locating a well tool sized and adapted for passage through relativelysmall diameter production tubing and having a source of fast neutrons atleast some of which having sufficient energy to cause the nuclearreaction 0¹⁶ (n,p) N¹⁶ and at least two gamma ray detectorslongitudinally spaced from said source and each other to a positionadjacent above or below the producing perforations to be investigated bypassing said tool through the production tubing while keeping the wellon production; b. repetitively irradiating the borehole environs withbursts of fast neutrons from said source; c. detecting substantiallybetween said neutron bursts at each of said detectors gamma raysprimarily caused by the decay of the unstable isotope N¹⁶ and generatingsignals representative thereof; and d. combining said signals accordingto a predetermined relationship to derive an indication of the directionand linear flow rate of any elemental oxygen nuclei comprising undesiredfluid flow in a preferred direction behind the well casing at saidadjacent position.
 2. The method of claim 1 and further including thestep of estimating the radial distance from the detector locations ofsuch undesired fluid flow and combining said distance and said linearflow rate indication according to a second predetermined relationship toprovide an indication of the volume flow rate of such undesired flow. 3.The method of claim 1 wherein the detecting step further includes thestep of separating said representative signals into at least twoseparate energy dependent signals.
 4. The method of claim 3 and furtherincluding the step of combining said at least two separate energydependent signals according to a third predetermined relationship toderive an indication of the radial distance from the detector to thelocation of the undesired behind casing fluid flow.
 5. The method ofclaim 4 and further including the step of combining said linear flowrate indication and said distance indication according to a fourthpredetermined relationship to provide an indication of the volume flowrate of the undesired behind casing fluid flow.
 6. The method of claim 1and further including the steps of:removing said well tool from theborehole after making the first measurement sequence adjacent above orbelow the perforation; reversing the juxtaposition of said source andsaid detectors; locating said tool in the opposite sense adjacent belowor above the perforations; and repeating steps (b), (c) and (d) withsaid tool in the new location.
 7. The method of claim 6 and furtherincluding the step of estimating the radial distance from the detectorlocations of such undesired fluid flow and combining said distance andsaid linear flow rate indication according to a second predeterminedrelationship to provide an indication of the volume flow rate of suchundesired flow.
 8. The method of claim 6 wherein the steps ofirradiating and detecting are performed by repetitively irradiating theborehole environs with bursts of fast neutrons and the detecting step isprimarily performed in the time intervals between the bursts.
 9. Themethod of claim 8 wherein the detecting step further includes the stepof separating said representative signals into at least two separateenergy dependent signals.
 10. The method of claim 9 and furtherincluding the step of combining said at least two separate energydependent signals according to a third predetermined relationship toderive an indication of the radial distance from the detectors to thelocation of the undesired behind casing fluid flow.
 11. The method ofclaim 10 and further including the step of combining said linear flowrate indication and said distance indication according to a fourthpredetermined relationship to provide an indication of the volume flowrate of the undesired behind casing fluid flow.
 12. A method formeasuring the direction, linear flow rate and volume flow rate of behindcasing water migration in a cased portion of a producing well operatingon gas lift and producing well fluid through casing perforations whichis transmitted to the earth's surface in relatively small diameterproduction tubing, without substantially interrupting the production ofthe well, comprising the steps of:a. lowering a well tool sized andadapted for passage through relatively small diameter production tubingand having a source of fast neutrons at least some of which havingsufficient energy to cause the nuclear reaction 0¹⁶ (n,p) N¹⁶ and atleast two gamma ray detectors longitudinally spaced from said source andeach other through the production tubing and into the producing zone ofthe well; b. moving said tool at a known rate upwardly or downwardlyfrom a position adjacent above or below said perforations to a positionapproximately at the same depth level in the well of the perforations;c. irradiating the borehole environs with fast neutrons from saidsource; d. detecting at each of said detectors gamma rays primarilycaused by the decay of the unstable isotope N¹⁶ and generating signalsrepresentative thereof; and e. combining said signals according to apredetermined relationship to derive an indication of the direction andlinear flow rate of any elemental oxygen nuclei comprising undesiredfluid flow in a preferred direction behind the well casing at saidadjacent position.
 13. The method of claim 12 and further including thestep of estimating the radial distance from the detector locations ofsuch undesired fluid flow and combining said distance and said linearflow rate indication according to a second predetermined relationship toprovide an indication of the volume flow rate of such undesired flow.14. The method of claim 12 wherein the steps of irradiating anddetecting are performed by repetitively irradiating the boreholeenvirons with bursts of fast neutrons and the detecting step isprimarily performed in the time intervals between the bursts.
 15. Themethod of claim 14 wherein the detecting step further includes the stepof separating said representative signals into at least two separateenergy dependent signals.
 16. The method of claim 15 and furtherincluding the step of combining said at least two separate energydependent signals according to a third predetermined relationship toderive an indication of the radial distance from the detectors to thelocation of the undesired behind casing fluid flow.
 17. The method ofclaim 16 and further including the step of combining said linear flowrate indication and said distance indication according to a fourthpredetermined relationship to provide an indication of the volume flowrate of the undesired behind casing fluid flow.
 18. The method of claim12 and further including the steps of:removing said well tool from theborehole after making the first measurement sequence moving said toolupwardly or downwardly; reversing the juxtaposition of said source andsaid detectors; lowering said tool into the producing zone through saidtubing string; and repeating steps (b) (c), (d), and (e) while movingsaid tool at a known rate in the opposite sense from that of the firstmeasuring sequence.
 19. The method of claim 18 and further including thestep of estimating the radial distance from the detector locations ofsuch undesired fluid flow and combining said distance and said linearflow rate indication according to a second predetermined relationship toprovide an indication of the volume flow rate of such undesired flow.20. The method of claim 18 wherein the step of irradiating and detectingare performed by repetitively irradiating the borehole environs withbursts of fast neutrons and the detecting step is primarily performed inthe time intervals between the bursts.
 21. The method of claim 20wherein the detecting step further includes the step of separating saidrepresentative signals into at least two separate energy dependentsignals.
 22. The method of claim 21 and further including the step ofcombining said at least two separate energy dependent signals accordingto a third predetermined relationship to derive an indication of theradial distance from the detector to the location of the undesiredbehind casing fluid flow.
 23. The method of claim 22 and furtherincluding the step of combining said linear flow rate indication andsaid distance indication according to a fourth predeterminedrelationship to provide an indication of the volume flow rate of theundesired behind casing fluid flow.